First off I will hands down go for the Sennheiser's. Why? Because that's what they do, they make headphones. B&W, they make speakers. Also, you need a headphone amp to drive these HD 700's or don't even bother. They will sound very FORGETTABLE without a great headphone amp (maybe a nice tubed one). The HD 700's have a impedance of 150 ohms and most headphone amps should work well (50 ohms is harder to drive, like planar headphones) So plan on making the plunge for a great headphone amp IMO one that costs around $500 to $1000 should pair nicely.

Sennheiser is known for making headphones that are fairly laid back sounding. I haven't heard the B&W's, but if you look at their speakers, I would expect them to sound very different from the 700's. So if you're biased to a certain type of sound, you need to be careful in your selection process.

Sennheiser also makes microphones, lots of 'em, so they are not a dedicated headphone company. That said, I love my Senn 600 phones. Most folks favor the darker, warmer 650 model. I have not heard the 700's. I have a very inexpensive headphone amp (120.00) which pleases me to no end.You should check out the threads at head-fi.org. That is a "headphone only" site and there's lots of good info there. Don't get overwhelmed.

Also, The Oppo does not have a amplifier inside it to drive headphones. It has a 1/4 inch jack so you can plug a set of headphones into it, but it will not drive a pair of high quality HD700's you need a external headphone amp.

I own the HD 700s by Sennheiser and like them very much. I also own AKG 7XXs and K701s in the past. They all sound very good using dedicated headphone amps. They aren't terrible using built in amps, like the from Oppo or other gear they just sound a lot better with the headphone amp. It is up to you to decide how much you want to spend but you really don't need to spend a lot to get decent sound.

"For a more intimate listening experience, a pair of headphones can be connected directly to the BDP-105s built-in headphone amplifiers. The headphone amplifier is connected directly to the ESS Sabre32 Reference DAC and offers a unique performance advantage over standalone headphone amplifiers."

Mofi is correct, as usual, certainly with respect to the 105 having a reasonably hefty built-in headphone amplifier. (I have no knowledge of its sonics, but based in part on his comments I'd expect it to be pretty good in that respect also).

As indicated on page 85 (pdf page 89) of the manual, the 105's headphone output can provide 63 mW (milliwatts) into the 150 ohm impedance of the HD700, which is way more than enough for anyone who isn't interested in having their hearing damaged. (Based on the 105 db/1 volt sensitivity and 150 ohm impedance of those phones, it can be calculated that 63 mW will drive them to an SPL of about 115 db).

There's no spec for the 105 corresponding to the 22 ohm impedance of the B&W P7, but the 105's output rating of 187 mW for a 32 ohm load would seem to indicate no problem at all in that case either (although I'd be hesitant to choose phones having significantly lower impedances than 22 ohms, given the lack of ratings into low impedances).

So the answer to "will I need to buy a headphone amplifier?" is no. Although it is not inconceivable that you might eventually PREFER to buy a headphone amplifier.

I own a lesser model of Bravo but can attest even the Bravo V2 (also a one tube design) sounds unbelievably good, even in comparison to my SinglPower Extreme! And they cost about $80- honestly its good.If you want what the Head-Fi community is excited about get a Garage1217 Project Ember.